Oscar de Marcos gave hosts Athletic the lead with the last kick of the first half but, in similar fashion to the Copa del Rey final between these two when Barca recovered from an early deficit to win 4-1, Pep Guardiola’s side bounced back impressively.
Xavi drew the treble-winners level in the 58th minute before Pedro Rodriguez smashed home a fine strike 10 minutes later to give Barca a 2-1 lead heading into next Sunday’s return meeting at the Nou Camp.
Unlike May’s cup final, when Athletic started strongly and took a 10th-minute lead through Gaizka Toquero, it was Barca who were quickest out of the blocks and they might have been four goals up inside the opening 30 minutes.
Bojan Krkic came close to breaking the deadlock in the fourth minute when his 25-yard drive was tipped onto the crossbar by Athletic goalkeeper Gorka Iraizoz, who soon after produced another superb one-handed save to deny Dani Alves from long range.
Barca midfielder Xavi, usually such a cool customer in the middle of the park, then suffered a rush of blood to the head as he blazed over from seven yards out with the goal at his mercy, before Bojan saw another chance go to waste.
Athletic rode their luck early on, but they gradually came back into the match and it was Barca’s turn to be pushed backwards towards the end of the half.
The home side’s first decent effort came in the 32nd minute when Aitor Ocio fired over from Andoni Iraola’s cross, a miss that left the big centre-back with his head in his hands.
Summer signing De Marcos went even closer four minutes later when he attempted an audacious 40-yard lob over Victor Valdes, forcing the Barca goalkeeper to back-pedal rapidly before tipping the ball over for a corner.
It was an impressive effort for no reward from De Marcos, but the rub of the green went the 20-year-old’s way on the stroke of half-time as he gave Athletic the lead.
De Marcos did well to skip inside Gerard Pique to fashion himself a shooting chance, but he then received a huge slice of luck as his left-footed shot took a crucial deflection off Carles Puyol before flying over the committed Valdes and into the roof of the net.
That was the last action of an entertaining first half and Athletic would have been hopeful at the break of holding some sort of lead for the second leg.
However, the Basque side had the upper hand for just 13 minutes of the second half before Barca drew level.
Seydou Keita’s cross beat everyone before reaching Pedro at the far post, and he cleverly feigned to shoot before sliding the ball inside to Xavi, who coolly slotted into the net.
Worse was to follow for Athletic as well, as soon after the same two Barca players combined to put their side ahead.
Xavi laid the ball off to Pedro 30 yards from goal, and the youngster took a few steps forward, easing past the challenge of Markel Susaeta in the process, before unleashing a superb swerving shot that flew past Iraizoz at his near post.
Pique and Keita had chances to increase the lead but failed to hit the target, while Iraizoz produced another fine stop to push Alves’ 30-yard thunderbolt onto the crossbar as Barca finished as they started – on top.